1. Field of the Invention
This invention relates to thermal printers and more specifically to coincident current activation of semiconductor mesas forming a thermal printhead when there are more mesas than x and y input drive lines.
2. Description of the Prior Art
In the past, semiconductor mesas of thermal printheads were typically activated by a single signal provided on a single conductor to each mesa. For example, a 5.times.7 mesa printhead required a total of 35 input lines.
More recently, the mesas of a thermal printhead have been provided with a latching heater circuit for providing a hot spot at the surface of the mesa, the latching circuit being responsive to a pair of coincident currents provided on a pair of intersecting coincident current lines, the mesas being arranged in a matrix requiring two signals for activation. In this manner, a 5.times.7 mesa array is activated by a total of 12 input drive lines.
This invention permits the use of a thermal printhead having more mesas than input drive lines by providing a decoder formed on a printhead with input drive lines connected to the input thereof and coincident current lines connected to the output. By properly coding the input drive lines, desired coincident current lines are selected to activate the appropriate latching heater circuits of desired mesas to form a selected character. For example, a thermal printer that has a 5.times.7 mesa thermal printhead may have the 7.times.9 mesa thermal printhead of the preferred embodiment of this invention substituted therefor without a need for increasing the number of input drive lines from 12 to 16.